Crabs are invertebrates that belong to the large group of animals known as crustaceans. A true crab possesses ten main limbs, placing it within the biological order Decapoda, which literally translates from Greek to “ten feet.” This defining characteristic of having five pairs of thoracic appendages is a standard feature across all true crabs, as well as their relatives like lobsters and shrimp.
The Decapod Standard: Five Pairs of Legs
The ten main appendages of a crab are organized into five distinct pairs, each arising from the fused head and thorax region, called the cephalothorax. The first pair is highly specialized and known scientifically as the chelipeds. These are the large pincers or claws that are often the most noticeable feature of a crab. The remaining four pairs of thoracic appendages are called pereiopods, which are the true walking legs. These pereiopods are primarily responsible for locomotion, allowing the crab to scuttle sideways across the substrate. The structure of these limbs is segmented, with each leg consisting of several jointed sections that permit a wide range of motion. Although the first pair, the chelipeds, looks and acts differently from the others, they are developmentally considered the first pair of legs.
Specialized Roles of the Claws
The chelipeds, or claws, are versatile tools. They are often sexually dimorphic, meaning they differ in size and shape between males and females, and sometimes even between the two claws on the same individual. In many species, such as fiddler crabs, the male develops one claw that is dramatically larger than the other, which can account for up to half of its body weight. This enlarged claw is used less for feeding and more for social communication, particularly in attracting a mate. Males will wave the oversized claw in an elaborate display to signal their fitness to females and to warn off rivals.
When engaging in competitive fighting, the robust claws are used as weapons for offense and defense against other crabs. Furthermore, the chelipeds are adapted for specific feeding behaviors, often differentiated into a “crusher” claw and a “cutter” claw. The heavier crusher claw is used to break open the shells of prey like mollusks, while the smaller cutter claw is used for more delicate tasks, such as tearing flesh or grooming.
Appendages That Are Not Legs
Beyond the ten legs, crabs possess numerous other specialized appendages. These structures are not counted among the legs but perform functions related to feeding, respiration, and reproduction. The maxillipeds are a set of three pairs of appendages located near the mouth, which have been modified to assist in feeding. These maxillipeds manipulate food, moving it toward the mouthparts and often filtering suspended particles from the water. In true crabs, the third pair of maxillipeds is typically broad and flattened, serving to cover and protect the more delicate mouthparts beneath.
Crabs also have pleopods, commonly known as swimmerets, which are attached to the segments of the abdomen. While these are used for swimming in some related crustaceans, in most true crabs, they serve other functions. In females, the pleopods are used to carry and aerate the fertilized eggs until they hatch, while in males, the first two pairs are often modified for reproductive purposes.